1. Field of the Invention
This invention relates to processes and compositions which accomplish at least one, and most preferably all, of the following related objectives when applied to formed metal surfaces, more particularly to the surfaces of cleaned aluminum and/or tin plated cans: (i) reducing the coefficient of static friction of the treated surfaces after drying of such surfaces, without adversely affecting the adhesion of paints or lacquers applied thereto; (ii) promoting the drainage of water from treated surfaces, without causing "water-breaks", i.e., promoting drainage that results in a thin, continuous film of water on the cans, instead of distinct water droplets separated by the relatively dry areas called "water-breaks" between the water droplets; and (iii) lowering the dryoff oven temperature required for drying said surfaces after they have been rinsed with water.
2. Discussion of Related Art
The following discussion and the description of the invention will be set forth primarily for aluminum cans, as these represent the largest volume area of application of the invention. However, it is to be understood that, with the obviously necessary modifications, both the discussion and the description of the invention apply also to tin plated steel cans and to other types of formed metal surfaces for which any of the above stated intended purposes of the invention is practically interesting.
Aluminum cans are commonly used as containers for a wide variety of products. After their manufacture, the aluminum cans are typically washed with acidic cleaners to remove aluminum fines and other contaminants therefrom. Recently, environmental considerations and the possibility that residues remaining on the cans following acidic cleaning could influence the flavor of beverages packaged in the cans has led to an interest in alkaline cleaning to remove such fines and contaminants. However, the treatment of aluminum cans with either alkaline or acidic cleaners generally results in differential rates of metal surface etch on the outside versus on the inside of the cans. For example, optimum conditions required to attain an aluminum fine-free surface on the inside of the cans usually leads to can mobility problems on conveyors because of the increased roughness on the outside can surface.
Aluminum cans that lack a low coefficient of static friction (hereinafter often abbreviated as "COF") on the outside surface usually do not move past each other and through the trackwork of a can plant smoothly. Clearing the jams resulting from failures of smooth flow is inconvenient to the persons operating the plant and costly because of lost production. The COF of the internal surface is also important when the cans are processed through most conventional can decorators. The operation of these machines requires cans to slide onto a rotating mandrel which is then used to o transfer the can past rotating cylinders which transfer decorative inks to the exterior surface of the cans. A can that does not slide easily on or off the mandrel can not be decorated properly and results in a production fault called a "printer trip". In addition to the misloaded can that directly causes such a printer trip, three to four cans before and after the misloaded one are generally lost as a consequence of the mechanics of the printer and conveyor systems. Jams and printer trips have become increasingly troublesome problems as line speed have increased during recent years to levels of about 1200 to 1500 cans per minute that are now common. Thus, a need has arisen in the can manufacturing industry, particularly with aluminum cans, to modify the COF on the outside and inside surfaces of the cans to improve their mobility.
An important consideration in modifying the surface properties of cans is the concern that such modification may interfere with or adversely affect the ability of the can to be printed when passed to a printing or labeling station. For example, after cleaning the cans, labels may be printed on their outside surface, and lacquers may be sprayed on their inside surface. In such a case, the adhesion of the paints and lacquers is of major concern. It is therefore an object of this invention to improve mobility without adversely affecting adhesion of paints, decorating inks, lacquers, or the like.
In addition, the current trend in the can manufacturing industry is directed toward using thinner gauges of aluminum metal stock. The down-gauging of aluminum can metal stock has caused a production problem in that, after washing, the cans require a lower drying oven temperature in order to pass the column strength pressure quality control test. However, lowering the drying oven temperature resulted in the cans not being dry enough when they reached the printing station, and caused label ink smears and a higher rate of can rejects.
One means of lowering the drying oven temperature would be to reduce the amount of water remaining on the surface of the cans after water rinsing. Thus, it is advantageous to promote the drainage of rinse water from the treated can surfaces. However, in doing so, it is generally important to prevent the formation of surfaces with water-breaks as noted above. Such water-breaks give rise to at least a perception, and increase the possibility in reality, of non-uniformity in practically important properties among various areas of the surfaces treated.
Thus, it is desirable to provide a means of improving the mobility of aluminum cans through single filers and printers to increase production, reduce line jammings, minimize down time, reduce can spoilage, improve or at least not adversely affect ink laydown, and enable lowering the drying oven temperature of washed cans.
In the most widely used current commercial practice, at least for large scale operations, aluminum cans are typically subjected to a succession of six cleaning and rinsing operations as described in Table 1 below. (Contact with ambient temperature tap water before any of the stages in Table 1 is sometimes used also; when used, this stage is often called a "vestibule" to the numbered stages.)
TABLE 1 ______________________________________ STAGE ACTION ON SURFACE NUMBER DURING STAGE ______________________________________ 1 Aqueous Acid Precleaning 2 Aqueous Acid and Surfactant Cleaning 3 Tap Water Rinse 4 Mild Acid Postcleaning, Conversion Coating, or Tap Water Rinse 5 Tap Water Rinse 6 Deionized ("DI") Water Rinse ______________________________________
It is currently possible to produce a can which is satisfactorily mobile and to which subsequently applied inks and/or lacquers have adequate adhesion by using suitable surfactants either in Stage 4 or Stage 6 as noted above. Preferred treatments for use in Stage 6 are described in U.S. Pats. 4,944,889 and 4,859,351, and some of them are commercially available from the Parker+Amchem Division of Henkel Corporation (hereinafter often abbreviated as "P+A") under the name "Mobility Enhancer.TM. 40" (herein often abbreviated "ME-40.TM.").
However, many manufacturers have been found to be reluctant to use chemicals such as ME-40.TM. in Stage 6. In some cases, this reluctance is due to the presence of a carbon filter for the DI water (normal Stage 6) system, a filter that can become inadequately effective as a result of adsorption of lubricant and surface conditioner forming additives such as those in ME-40.TM.; in other cases, it is due to a reluctance to make the engineering changes necessary to run ME-40.
For those manufacturers that prefer not to add any lubricant and surface conditioner material to the final stage of rinsing but still wish to achieve the advantages that can be obtained by such additions, alternative treatments for use in Stage 4 as described above have been developed and are described in U.S. Pat. Nos. 5,030,323 and 5,064,500. Some of these materials are commercially available from P+A under the name FIXODINE.TM. 500.
However, the reduction in coefficient of friction provided by prior art treatments in either Stage 4 or Stage 6 can be substantially reduced, often to an unacceptable level, if the treated cans are subjected to extraordinary heating after completion of the six process stages described above. Such extraordinary heating of the cans in the drying oven occurs whenever a high speed production line is stalled for even a few minutes, an event that is by no means rare in practice. In practical terms, the higher COF measurements correlate with the loss of mobility, thereby defeating the purpose of introducing mobility enhancing surfactants into can washing formulations. Accordingly, it is an object of this invention to provide means of improving the mobility of aluminum cans and/or one of the other objects stated above that are superior to means taught in the prior art, particularly with respect to stability of the beneficial effects to heating well beyond the minimum extent necessary for drying the treated surfaces.